1. Field of the Invention
This invention relates to a method for drawing a glass ingot wherein the glass ingot is drawn to a predetermined outer diameter to obtain a glass rod or preform for an optical fiber having the predetermined outer diameter.
2. Description of the Prior Art
For the drawing of a large-sized glass ingot having a diameter exceeding 100 mm, there is usually used a method wherein the ingot is heated and softened in an electric furnace, under which it is drawn by application of a given tension thereto. Such a large-sized ingot is schematically shown in FIG. 4. In the figure, an ingot 10 has a body 12 and tapered portions 14, 14' at opposite sides thereof. The tapered portions 14, 14' are, respectively, connected to dammy portions 16, 16'.
For the drawing, the ingot 10 is set in position of an electric furnace 18 having a heating unit 20 provided to surround the ingot 10 therearound as shown in FIG. 5. Reference numeral 22 indicates take-up rollers.
When the ingot 10 having a diameter of D1 is fed at a rate of V1 and drawn to a diameter of D2 at a take-up rate V2, the following relationship is established EQU D1.sup.2 .times.V1=D2.sup.2 .times.V2.
In this way, a preform having the diameter of D2 can be obtained. In this connection, however, when the drawing proceeds to a final stage where an upper tapered portion is heated, this portion is more likely to heat to a temperature higher than the body portion 12 since the diameter of the tapered portion 14 is gradually reduced. Accordingly, when a given quantity of heat is invariably applied to the ingot 10, the upper tapered portion 14 is much more quickly drawn, so that a portion 12a of the body portion 12 contiguous to the tapered portion 14 is drawn into a preform whose diameter is larger than an intended one. This is particularly shown in FIG. 6. More particularly, the portion 12a cannot be drawn to a given outer diameter because of the ease in the drawing at the tapered portion 14, but with a relatively great increase in the diameter as shown.
To avoid this, many attempts have been made wherein while the outer diameter of the tapered portion is monitoring, the data of the outer diameter is fed back to a control unit, but without success.